1. Field of the Invention
The present invention relates to a motor drive device for driving an electric motor, more specifically, relates to a motor drive device having a drive circuit for driving a power supply relay and a phase relay providing a fail-safe function.
2. Description of Related Art
Japanese Laid-Open Patent Application Publication No. 2011-244611 discloses a motor drive device using a Pulse Width Modulation (PWM) control method. This motor drive device has an inverter circuit that is driven by a PWM signal having a predetermined duty, and a fail-safe circuit provided at a position between the inverter circuit and a motor.
The inverter circuit is constituted by a so-called three-phase bridge, in which each switching element performs on-off operation according to the PWM signal, to thereby supply a drive voltage from the inverter circuit through the fail-safe circuit to each phase of the motor. When, for example, an abnormality occurs, the fail-safe circuit shuts off supply of electricity from the inverter circuit to the motor to stop the motor, thus performing a fail-safe function. As the switching elements constituting the inverter circuit and the fail-safe circuit, there is disclosed an example employing N-channel MOSFETs.
In the motor drive device disclosed in the abovementioned patent document, the MOSFETs of the fail-safe circuit are individually on-off controlled by a fail-safe drive unit. Furthermore, as the power supply relay, a mechanical relay is employed, which is open-close controlled by a control unit. Although the patent document discloses that a semiconductor switching element may be employed instead of the mechanical relay, the circuit construction is one based on the premise that a mechanical switch is employed, and that the mechanical switch is controlled separately from the MOSFETs of the fail-safe circuit.
However, if the MOSFETs of the fail-safe circuit are configured to be individually on-off controlled, the circuit construction of the fail-safe drive unit becomes complicated. Furthermore, since the fail-safe drive unit is provided discretely from the control unit, there occurs a problem that the number of components increases and cost-saving becomes difficult.